The invention relates to an electroacoustic transducer with the features specified in the introductory part of claim 1. Electroacoustic transducers of this type consist of a dome-shaped carrier which on its inner side is furnished with piezoelectric elements which in each case are electrically contacted on a side directed to the carrier as well as a side distant from the carrier and connected to a high voltage impulse generator. On applying a voltage by way of the piezoelectric effect a sound wave in the form of a pressure wave is produced, which on account of the dome-shaped arrangements of the piezoelectric elements is focussed to a point or a region which is dependent on the radius of curvature of the carrier. With such electroacoustic transducers, apparatus for medical treatment, for example lithotripters are equipped whose regions of applications are numerous (from the destruction of stones to the treatment of soft tissue and pain).
An advantage of these piezoelectric transducers with a dome-shaped carriers and piezoelectric elements arranged thereon lies in the high energy intensity in the focus region which can hardly be reached by other systems, by which means a directed and largely pain-free application of shock waves may be effected. The complete irradiated sound power however on account of the limited available irradiation surface may not be increased further beyond certain limits. An enlargement of the irradiating surface can specifically only be achieved by enlarging the radius of the dome or by enlarging the opening angle (aperture). The first is in practise not possible since with this the constructional size of the sound source would increase so much that the application in the field of table apparatus would no longer be possible. An enlargement of the aperture angle can likewise be ruled out since otherwise the penetration depth of the shock waves in the patient itself would become so slight and any increase in power would be compensated by this.
On the other hand one strives rather to reduce in size the constructional shape of the transducer in order to reduce the constructional size of the therapy apparatus and thus to form new fields of application, for example treatment of salival stones or treatment of pain. The acoustic power of the transducer may be increased by electrically precharging the piezoeclectric elements but this increase in power is at the expense of the life expectancy of the transducer dome.
From DE 31 19 295 it is known to arrange several electrical transducers behind one another and to so trigger these in a delayed manner that their pressure waves add. Such an arrangement has been found to be unsuitable with the application of piezoelectric transducer domes, since the focus region in the direction of the treatment depth is heavily extended which is undesirable. Furthermore it has been found out that enormous losses are caused by damping which the sound waves undergo when they must penetrate through a prior-mounted transducer. Finally also these inner friction losses are at the expense of the life expectancy, in particular of the foreward transducer dome directed towards the treatment location.